A microfabrication technique is a very important basic technique in the field of semiconductor technology, and the research and development thereof have been progressed for further finer microfabrication. In recent years, in particular, the degree of request for microfabrication technique is becoming more than ever with respect to the high integration of a large scale integration circuit because of finer circuit patterns and finer wiring a pattern, a finer pattern of contact holes for interlayer wiring of a cell, and the like.
In view of such a circumstance, in the field of a photo mask production technique used in a photolithography step in the microfabrication as described above, a technique for writing a fine and correct circuit pattern (mask pattern) also becomes being demanded in the field of a photo mask production technique.
In order to form a highly precise mask pattern, it is required to form a highly precise resist pattern on a photo mask blank. Generally, reduction projection is performed when forming a pattern on a semiconductor substrate by photolithographic technique. The size of the pattern formed on the photo mask is therefore approximately four times larger than the side of the pattern formed on the semiconductor substrate. However, this does not mean that the desired precision of the pattern formed on the photo mask is smaller than the pattern formed on the semiconductor substrate. Rather, the precision of a pattern formed on the photo mask as a master disc is desired to be more than an actual pattern obtained after exposure.
In today's photolithography technical field, the size of a circuit pattern to be drawn is considerably smaller than the wavelength of light to be used for exposure. Thus, in the case of forming a photo mask pattern with a just four-times larger circuit pattern, light interference or the like, which is generated under exposure, influences on transfer of an original shape. As a result, the original shape cannot be transferred onto the resist film of a photo mask blank.
In order to reduce such influence, therefore, it may be necessary to process a photo mask pattern into a shape more complicated than the actual circuit pattern. The shape may be, for example, a shape subjected to optical proximity correction (OPC).
Accordingly, a more precision processing technique has been also desired in a lithographic technique for forming a photo mask pattern. In some cases, lithography performance is expressed in resolution limit. As described above, however, a pattern formed on a photo mask as a master disc desires more precision than an actual pattern formed after exposure. Thus, resolution limit required for formation of a photo mask pattern is almost equal to or more than one required in lithography for forming a pattern on a semiconductor base.
In general, when forming a photo mask pattern, a resist film is formed on the surface of the photo mask blank in which a light-shielding film is mounted on a transparent substrate, and a pattern is then drawn (exposed) on the resist film by an electron beam. Subsequently, after obtaining a rest pattern after developing the exposed resist film, the light-shielding film is etched by using this resist pattern as a mask to obtain a light-shielding (film) pattern. The light-shielding (film) pattern thus obtained is served as a photo mask pattern.
In this case, the above resist film should be thinned depending on the degree of fineness of the light-shielding pattern. This is because, when forming a fine light-shielding pattern while keeping the thickness of the resist film, the ratio (aspect ratio) of the thickness of the resist film to the size of the light-shielding pattern becomes large and causes troubles of unsatisfied pattern transfer due to degraded shape of the resist pattern, collapse or peel of the resist pattern, or the like.
As a material of the light-shielding film mounted on the transparent substrate, many kinds of materials have so far been proposed. Among them, however, a chromium compound has been practically used because of much know-how on etching, for example.
Dry etching of a chromium-containing material film is generally performed by chlorine-containing dry etching. In many cases, however, chlorine-containing dry etching has a certain level of ability to etch an organic layer. In the case that a resist pattern for etching a light-shielding film is formed on a thin resist film, therefore, the resist pattern is etched too much to ignore by chlorine-containing dry etching. As a result, the proper resist pattern cannot be correctly transferred to the light-shielding film.
Hence, a resist material having excellent etching resistance has been requested. In practice, however, such a resist material has not been known yet. For this reason, to obtain a light-shielding (film) pattern having high resolution property, a light-shielding film material having more processing accuracy has been reexamined.
As a specific effort to reexamine the light-shielding film material having more processing accuracy, there is reported an attempt to increase the etching rate of a light-shielding film by allowing a chromium compound serving as a light-shielding film material to contain only a predetermined amount of a light element (see, for example, Patent Literature 1 and Patent Literature 2).
Patent Literature 1 (WO 2007/74806 A) discloses a technique that uses a material mainly containing chromium (Cr) and nitrogen (N) and having an X-ray diffraction peak of substantially CrN (200) as a light-shielding film material to suppress a decrease in thickness of a resist film by increasing the dry etching rate of the light-shielding film.
Furthermore, Patent Literature 2 (JP 2007-33470 A) discloses the invention of a photo mask blank where the composition of a light-shielding film formed of a chromium-containing compound is made rich in light element and low in chromium composition as compared with the composition of the conventional film so that the composition, film thickness, and laminated structure of the photo mask blank can be suitably designed to obtain desired transmittance T and reflectance R while trying to increase the dry etching rate of the light-shielding film.